1. Field of Invention
The invention relates to an inorganic fluorescent material and a method for preparing the same, and more particularly relates to an inorganic nitride-based fluorescent material in which a metal oxide, a metal hydroxide, or a metal carbonate is coated on a surface of a nitride-based fluorescent host material.
2. Description of Related Art
A light-emitting diode (LED) features compact volume, rapid response speed, low power consumption, long lifetime, and high reliability. The LED has been used as an indicator lamp and applied in a display panel from the beginning. With development of white LED, the LED now has been widely applied for illumination.
Currently, the white LED utilizes a blue LED chip emitting a blue light to excite electrons in a yellow phosphor (e.g., YAG: Ce3+), such that part of the blue light emitted by the blue LED chip is converted into a broad band consisting mainly of yellow light. By mixing the unconverted blue light and the yellow light, which can stimulate the red-light receptor and the green-light receptor in human eyes, a white light is presented in the eyes. However, the application of the yellow phosphor is limited by a high correlated color temperature (about 4,500 to 11,000 K) and low color-rendering index (CRI) thereof.
Accordingly, to obtain a warm white light of high quality, a nitride material that can produce red light can be applied as a phosphor. Since the nitride-based fluorescent material is capable of emitting a red-light broad band, the CRI of the white LED can be improved. Further, the nitride-based fluorescent material has excellent environmental stability, non-toxicity and high light emitting efficiency, and is applicable to the white LED device.
Nonetheless, in operation of the LED device, a LED chip generates heat causing a temperature rise through the entire LED device. The elevated temperature reduces the light-emitting efficiency of the fluorescent material referred to as “thermal quenching”. As such, the thermal stability of the fluorescent material becomes poor and influences the light-emitting efficiency of the LED device, and even reduces the service life of the LED device.
Therefore, to address the aforesaid problems, it is desired to develop an inorganic nitride-based fluorescent material, which is applicable to a white LED device, which can not only provide a great luminance but also has excellent thermal stability, so as to prolong the service life of the LED device.